1. Field of the Invention
This invention relates to a correlator useful in a vehicular collision avoidance system.
2. Description of the Prior Art
Collision avoidance systems have been proposed as means of preventing collisions by vehicles both of the maritime vessel type and of the airborne type. Cooperative systems of the asynchronous type include interrogator-transponder techniques in which interrogation signals or probes are transmitted from one vehicle on a random basis. Vehicles in the vicinity receiving such probes respond with signals of various indicia to provide to the interrogation vessel information relating to both range and information identifying the vessel and its position. In airborne systems such information may include the relative or actual altitude of the aircraft.
An existing collision avoidance system known by the acronym SECANT (Separation Control of Aircraft by NonSynchronous Techniques) employs probes identified by any one of a plurality of frequencies and replies using different frequencies of the same band but arranged into a predetermined correspondence to a particular probe frequency. Special correlation techniques separate the true reply received by any one vehicle from received reply signals induced by probes from remote vehicles, the latter replies being identified as fruit. Such a system is described in U.S. Pat. No. 3,755,811 issued Aug. 28, 1973, and U.S. Pat. No. 3,803,608 issued Apr. 9, 1974, based on the inventions of Jack Breckman.
A full range correlator of the type described in U.S. Pat. No. 3,887,916 issued June 3, 1975, to R. B. Goyer for "Correlator and Control System for Vehicular Collision Avoidance" allows signals representing a target to bridge or straddle adjacent correlator bins. In general, the correlator disclosed in the Goyer patent provides for receiving signals after thresholding that are in the order of 1.2 to 1.5 microseconds wide. For a system in which the correlator bins are 500 feet in range, i.e., 1.0 microseconds wide, the received signals being phased with respect to the range bins as a function of the distance of the replying vehicle, it is possible for a strong return to be coincident with portions of two or three bins. Further, the addition of system noise introduces a phase jitter of about 0.1 microseconds so that the portion of the bin that is occupied by a given reply may vary from one reply signal to the next reply signal. The Goyer correlator (patent, supra) operating at a 10 MHz sampling rate gererates 10 samples during each bin period. The number of segments of bins that are thus occupied by a replied pulse is accumulated in the counter representing that 500 foot range bin. For an operation of such a correlator utilizing a program of 25 pulses to correlate replies for reliable and accurate target identification, a maximum of 250 counts can be accumulated for a series of replies coincident with a particular 500 foot range bin. The condition that is termed "failed to correlate" is identified by a preselected threshold arranged in one of the SECANT system applications to operate at less than 100 out of 250 counts. A reply pulse occupying only four of the 25 sampled periods in a bin would correlate if there were, in fact, no missing replies. A pulse occupying five periods could be missing five of the 25 replies, and yet fewer replies will correlate as a larger portion of the bin is occupied. For such a system, only 10 replies are required for a pulse occupying the complete 500 foot bin. One implementation of the correlator described in the above-identified Miller "Correlator," copending application, Ser. No. 569,995, uses long shift registers in a multiple-bin, parallel counter array, with a "failed to correlate" threshold of 10 out of 25. Since a bin of such a correlator is sampled only for occupancy during any portion of the microsecond to effect a count of one, a pulse of 1.2 to 1.5 microseconds could correlate in two or three adjacent bins. A "hand-off" of one target to two or three tracker assignments could seriously reduce the capability of a tracker for multiple targets such as described in the copending applications of Miller, Ser. No. 643,481 and Ross, Ser. No. 643,478 if such a hand-off occurred. It appears that the individual 500 foot bin could be sampled at a 10 MHz rate and the count entered in a range parallel counter register to obtain the same performance. Such a modification however would require significantly more hardware. It would appear that the result of a 10 MHz sample count rate could be thresholded whereby only a count of "1" be entered in the correlator shift register array. For such an arrangement, only the small precounter operating at 10 MHz would be required, which is only a small increase in hardware.